1. Field of the Invention
The invention relates to an illumination unit for an optical system, including an illumination system which comprises, in this order, a radiation source unit, a first optical integrator, and a detection system including a radiation-sensitive detector for measuring an illumination dose.
The invention also relates to an optical apparatus for step and/or scan-imaging a mask pattern on a substrate, which apparatus is provided with such an illumination unit.
2. Description of the Prior Art
An illumination unit of the type described in the opening paragraph is known, for example, from U.S. Pat. No. 5,343,270. Such an illumination unit may be used, for example, in a wafer stepper. A wafer stepper is an optical apparatus for optically and repetitively imaging a mask pattern in a photosensitive layer which is present on a substrate, for example, a silicon substrate and which is used for manufacturing integrated circuits (ICs). A projection lens system images the pattern on a first sub-area, or field, of the substrate. Wafer steppers can be distinguished into steppers and scanners. In a stepper, the mask pattern is imaged on a field in one run. Subsequently, the substrate is moved along an accurately defined distance, whereafter the pattern is imaged on a subsequent substrate field, after which the substrate is moved again. This operation is repeated until the mask pattern has been imaged on the desired substrate fields. In optical lithography, one of the objects is to image smaller details, so that more components can be accommodated on an IC. For this purpose, a projection lens having a larger numerical aperture (NA) must be used, which has, however, a smaller depth of focus. Another object is to illuminate larger fields, so that the number of components per IC can also be raised in that way. For this purpose, the projection lens must have a larger image field. The requirements of a larger NA and a larger image field contradict each other which, in practice, can only be fulfilled to a certain extent with a great deal of trouble and at a high cost, and are altogether impossible in the case of even further enlargements. It has therefore been proposed to image the mask pattern in a scanning mode. In this mode, a part of the mask pattern is imaged each time on a corresponding part of a substrate field. To this end, only a part of the mask pattern is illuminated, and the mask and the substrate are moved synchronously with respect to the projection lens and the illumination beam until the entire mask pattern has been imaged on the substrate field. When a full field has been illuminated, for example, the substrate is moved along an accurately defined distance, similarly as in a stepper, so that the next field is reached.
In order to form well-defined patterns in the substrate, this substrate should be illuminated with an accurately determined quantity of energy, hereinafter referred to as dose. Moreover, the illumination of a substrate field should be performed as rapidly as possible, so that a maximal number of substrates can be illuminated per unit of time. It is therefore of great importance that the actual dose is measured in an accurate and reliable manner, so that it can be rendered equal to the desired dose as accurately as possible.
To measure the dose, a radiation-sensitive detector is arranged in the radiation path of the illumination beam, behind a partly transmissive folding mirror in the illumination system described in U.S. Pat. No. 5,343,270. The detection unit described in U.S. Pat. No. 5,343,270 does not allow for the fact that, dependent on the mode of operation, one wants to have the facility of changing the energy distribution within the illumination beam in the current step and/or scanning photolithographic apparatuses.
Since the active surface area of the detector with which the measurement takes place is generally relatively small, the distribution of light at the area of this surface should be very homogeneous, and a maximal quantity of radiation should reach the detector.